This invention relates to a process for in-situ mining of copper and nickel from a deep subterranean ore body using a stable, two phase lixiviant. More specificially, it relates to a modification of the process set forth in copending patent application Ser. No. 724,548 entitled In-Situ Mining Method and Apparatus filed on even date herewith, the teachings of which are incorporated herein by reference.
There is known to be located through out various regions of the globe, large, deep lying deposits of copper in the form of low grade porphyry ores. A porphyry copper deposit is one in which the copper bearing minerals occur in disseminated grain and/or in veinlets through a large volume of rock such as schist, silicated limestone, and volcanic rock. Acid igenous intrusive rocks are usually in close association. The deposits are typically large tonnage but low grade, and have an average copper concentration of less than about 1 percent. Copper minerals found in these deposits are usually sulfides and most commonly are chalcopyrite. There are also massive sulfide deposits treatable by the present invention which are deep seated and contain discrete blebs of nickel sulfide, copper sulfide, or copper-nickel sulfide in association with iron sulfide. A representative list of minerals which can be treated to recover copper using the process of this invention includes chalcocite, digenite, covellite, pentlandite, heazlewoodite, vaesite, and violarite.
When a deposit of the type described is of sufficiently high grade and outcrops on the surface or is sufficiently close to the surface, the ore may be mined by open pit methods, and the metal values separated from the gangue constituents by techniques such as floatation.
Deeply buried or very low grade copper deposits cannot be easily exploited. Conventional open pit mining is not available because the costs involved are prohibitive and because landscape destructive open pit mining techniques have been restricted in many areas.
It has been proposed to extract the copper from the deeply buried porphyry deposits by in-situ leaching techniques. With in-situ mining, a hole is drilled and a leach liquor is pumped down the hole into the ore containing the metal to be recovered. After the liquor has leached the metal values, it is brought back to the surface and the values are recovered.
There are many prior art procedures for in-situ mining. Most of these procedures, however, involve rubblizing the ore which is to be leached by explosive methods. In contrast, the process and apparatus disclosed in the aforementioned copending Ser. No. 724,548 and the instant invention involve leaching the copper in-situ, without rubblizing the ore by employing a two-phase lixiviant comprising very small oxygen containing gas bubbles admixed with leach liquor. For this method to be successful, the oxygen containing bubbles must be small enough to penetrate the natural fracture openings within the rock so that the sulfidic minerals may be oxidized. Thereafter, the leach liquor solubilizes the metal values.
Prior to the present invention and prior to the process disclosed in the aforementioned copending U.S. application Ser. No. 724,548 two-phase lixiviants useful for such purposes, although theoretically desirable, were thought to be unattractive for a number of reasons. The two primary problems were the size of the bubbles and the difficulties stemming from phase separation, i.e., formation of gas pockets. So severe were the problems associated with such two-phase in-situ mining procedures, that research in this area has been discouraged.
The types of copper and nickel bearing ores with which the present invention is concerned generally have a porosity of about 3% and are found on the order of 2,000 feet below the surface. The cracks, pores, and other fracture openings in these rocks usually have dimensions on the order of 10 to 300 microns. Since these openings contain the metal values of interest, and since the sulfidic minerals in which the metal values are contained must be oxidized before the leach liquor can take effect, it is necessary that the lixiviant contain a high concentration of oxygen containing bubbles small enough to move freely through the openings. The ideal lixiviant would comprise a leach liquor containing a high concentration of oxygen in the form of stable bubbles having a diameter less than about 10 microns. It is also desirable that the lixiviant have a viscosity at the temperature of use of close to 1.0 centipose.
In practice, as mentioned above, such a two-phase lixiviant has been difficult to produce and even more difficult to maintain. Methods are known for forming small bubbles in a liquid, but in the known two-phase systems, the bubbles tend to coalesce, form larger bubbles, and ultimately form large pockets of gas. The higher the viscosity of the liquid phase, the easier it is to form bubbles of the size described and to maintain them. In systems using low viscosity compounds with the high volume fraction of gas, i.e., higher than 15-30%, production and maintenance of small gas bubbles becomes very difficult. Another variable which affects the stability of gas bubbles in a two-phase system is the flow rate of the lixiviant. Generally, it has been observed that the higher the flow rate, the easier it is to maintain the gas-liquid dispersion. However, in use, there is an upper limit in the flow rate which limit makes unavailable any advantage which might theoretically be gained by employing a high flow rate. When deterioration of the two-phase system occurs in the in-situ mining procedure such as that disclosed in the aforementioned copending application, a two-phase lixiviant is rendered inoperative or unacceptably inefficient.